Due to its unique combination of relatively low cost, low toxicity and high radiation absorption capacity, boron has long been used for shielding of neutron radiation in connection with nuclear reactors. Boron has a high absorption cross section, a meaning that the probability is high that a neutron passing through a sheet or layer containing boron atoms will interact with a boron nucleus and hence be prevented from passing through. Native boron contains both B.sup.10 and B.sup.11 isotopes, the former having a fivefold higher neutron absorption capacity than the latter. Through an enrichment process, the B.sup.10 content can be increased above the naturally occurring level of about 19 atomic percent.
Generally, radiation shields using boron are in the form of a composite material. This is necessitated by the inability of boron, or compounds thereof (e.g., boron carbide), to provide minimum acceptable levels of thermal shock resistance, fracture toughness and tensile strength. Incorporating boron into a composite can provide greater control of the physical properties of the shield since the non-boron components can be selected for strength or other important properties rather than their radiation shielding effect. Despite this greater control, however, prior art boron composites have suffered from many drawbacks.
It is known, for example, to mix isotopic B.sup.10 (i.e., highly enriched boron) with molten aluminum and then cast it as an ingot. The ingot is then rolled to form a sheet. During the casting process there is significant clumping of the boron due to solidification effects. The clumped boron particles, due to their larger size, are prone to fracturing during the rolling step, which in turn causes smearing of the boron into "stringers," i.e., elongated boron particulates. This can be seen in FIG. 1, which is a photomicrograph of a boronated aluminum alloy formed using an ingot metallurgy technique. The large particles in the photomicrograph are clumped boron carbide particulates. During the rolling process, some of the clumps fracture and form stringers which can be in excess of 300 .mu.m in length. Void spaces are formed between the fractured particles which may lead to corrosion. Furthermore, the performance of the shield is degraded because of the loss of uniformity caused by clumping and subsequent fracturing.
Why this occurs can be explained in terms of the measurement of "nearest neighbor distance" between the boron particulates. In a highly uniform matrix, the distance between adjacent boron particulates falls within a narrow statistical distribution. Thus, throughout the matrix, the distance between a given boron particle and the nearest neighboring particle is relatively constant. Conversely, the more non-uniform the distribution, the greater the variance in nearest neighbor distance. The clumping of boron particulates during casting increases the non-uniformity due to migration of smaller particulates from more uniformly distributed positions into consolidated larger particulates. FIG. 2 is a histogram of nearest neighbor distances for the same boronated aluminum alloy of FIG. 1. As can be seen from FIG. 2, the nearest neighbor distances vary widely with a standard deviation of 14.95 .mu.m.
The effect of nearest neighbor distance on radiation absorption is as follows. Neutron radiation entering a matrix with uniform distribution of boron particulates will be absorbed at a uniform rate according to the statistical probability of the neutrons encountering a boron nucleus. However, for a non-uniform matrix, the neutron absorption is also non-uniform. In areas of the matrix where the nearest neighbor distances are small, for example, neutrons are successfully intercepted. Where the nearest neighbor distances are large, significant radiation may pass through because the statistical odds of encountering a boron nucleus are diminished. Since pass through of radiation over even small areas of the matrix is generally unacceptable, non uniform matrices have limited utility. Neutron absorption by non-uniform matrices can be enhanced by increasing the percentage of B.sup.10 in the particulates, however, highly enriched boron is expensive and significantly increases the cost of the radiation shielding. The absorption may also be increased by increasing the thickness of the shield. This solution also has drawbacks because the shield must fall within specified weight and size restrictions.
An alternative to casting and rolling is powder metallurgy. This technique holds the promise of providing highly uniform dispersions of boron particulates. Prior to the present invention, however, the full benefit of this technique had not been realized. Also, compromises in shield effectiveness were necessary in order to provide adequate adhesion of the powder particulates. U.S. Pat. No. 5,700,962, for example, describes metal matrix compositions formed by blending a metal matrix powder material with boron carbide powder. Included with the boron carbide powder are small amounts of silicon, iron and aluminum, which function as chelating agents by forming intermetallic bonds with the metal matrix material. The metal matrix material can be aluminum or an aluminum alloy. The powders are mixed, isostatically compressed, degassed, sintered and then extruded.
Prior to the sintering step, the ingots (billets) are heated to burn off binder and water. The composition of the binder is not disclosed, however, it is assumed to be organic in nature. This debinderizing step (designated S10 in FIG. 2 of the patent) results in microstructural discontinuities and limits the maximum size of the product. Although organic binders are fugitive, beyond an ingot diameter of about three inches, diffusion effects become significant enough to prevent complete removal of binder even at temperatures well above that of binder decomposition. If significant binder is left in the ingot, decomposition and subsequent generation of hydrogen gas can occur in later processing stages, e.g. welding, creating a hazardous explosive environment. Attempts to remove all the binder from large diameter billets results in long heating cycles and increased costs, as well as difficulties in controlling the debinderization conditions.
The use of chelating agents is also problematic because many of those mentioned in U.S. Pat. No. 5,700,962 can significantly degrade performance of the neutron shielding material. Iron in particular is very harmful because it transmutates to radioactive isotopes with longer half lives.
Another approach to radiation shielding is the product known as BORAL, manufactured by AAR Advanced Structures. This product is a composite plate material having a core of mixed aluminum and boron carbide particles with aluminum cladding on both sides. This structure cannot be welded and is furthermore subject to corrosion if the aluminum cladding is breached. Since the core is not full density, hydrogen gas can be rapidly generated in the presence of water and result in blistering of the skin/cladding layer.
Hence, there remains a need in the art for a radiation shielding composition which can be easily manufactured, is not subject to corrosion, can be formed into a variety of shapes, including structurally self supporting elements, can be welded, and which form strong metal particle-to-metal particle bonds without the need for binders or chelating agents.